Interleukin 1 and interleukin 2 are products of the immune system whose action has been almost exclusively studied in the modulation of immune cell growth. Essentially these molecules are considered paracrine hormones. Our hypothesis is that these proteins may be endocrine hormones whose action may be on cells on non-immune origin. We have seen that both IL-1 and IL-2 affect human breast cancer cell growth and further that this action occurs on hormone-dependent breast cancer cells, but not on hormone-independent breast cancer. These studies have been conducted in vitro using 3H-thymidine incorporation as an index of proliferation or actual cell number. Additionally, an IL-1 receptor has been characterized on MCF-7 breast cancer cells. In vivo IL-2 has been shown to suppress tumor cell growth using athymic nude mice implanted with tumor cells and IL-2 as the model system.